GI5 


Studies  on   Silver  Goulometer 
by 

0.   D.    Buckner. 


•N? 


UNIVERSITY  OF  CALIFORNIA 
AT  LOS  ANGELES 


8061  'IZ  NVf  1W 


japu.ig 


Studies   on 
the   Silver  Coulometer 


A     DISSERTATION 

PRESENTED    TO    THE 

FACULTY  OF    PRINCETON    UNIVERSITY 

IN  CANDIDACY  FOR  THE  DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 


G.    D.    BUCKNER 


ACCEPTED  BY  THE  DEPARTMENT  OF  CHEMISTRY 
JUNE,  1912. 


paper  presented  at  the  XXII  General 
Meeting  of  the  American  Electrochem- 
ical  Society,  in  Joint  Session  with 
Sections  II:  Inorganic  Chemistry,  Xa: 
Electrochemistry,  and  Xb:  Physical 
Chemistry,  of  the  VIII  International 
Congress  of  Applied  Chemistry,  in  New 
York  City,  September  9,  1912.  Dr.  W. 
Lash  Miller  in  the  Chair. 


STUDIES  ON  THE  SILVER  COULOMETER* 

By  G.   D.   BUCKNER. 

Introduction. 

Previous  to  the  starting  of  this  investigation  there  had  been 
extensive  studies  and  experiments  made  with  the  silver  cou- 
lometer,  dating  back  to  the  fundamental  work  of  Rayleigh  and 
Sedgwick1  in  1883,  when  the  coulometer  first  became  an  accurate 
instrument  for  the  measure  of  electric  current.  The  present 
investigation  was  undertaken  in  order  to  throw  light  upon  a 
matter  which  had  received  no  attention  in  the  numerous  articles 
published  on  the  silver  coulometer. 

In  the  discussion  which  followed  Duschak  and  Hulett's2  work 
on  the  silver  coulometer  the  point  was  made  by  Mr.  Hering3 
that,  since  platinum  is  known  to  contain  oxygen  in  some  form, 
it  seems  possible  that  dissolved  oxygen  may  play  some  part  in 
the  deposition  of  silver,  and  it  would  naturally  follow  that 
abnormal  weights  might  be  caused  thereby. 

Platinum  is  known  to  take  up  oxygen  either  in  solution  or  as  a 
compound,  forming  an  oxygen  electrode  in  equilibrium  with  the 
oxygen  in  the  electrolyte.  On  passing  a  current  through  a 
solution  of  silver  nitrate  the  main  reaction  at  the  cathode  would 
be  the  deposition  of  silver,  but  some  of  the  current  would  go  to 
the  removing  of  the  oxygen,  and,  since  8  grams  of  oxygen  are 
equivalent  to  107.88  grams  of  silver,  it  follows  that  there  might 
be  a  deficit  in  the  amount  of  silver  deposited,  an  amount  which 
might  easily  admit  of  measurement.  It  seemed  possible  that 

*  The  work  described  in  this  article  was  made  possible  by  a  grant  from  the 
Elizabeth  Thompson  Science  Fund,  which  placed  at  our  disposal  some  of  the  special 
apparatus  and  materials  needed  for  this  investigation,  and  the  authors  take  pleasure 
in  acknowledging  their  indebtedness  to  the  Trustees  of  the  Fund. 

1  The  literature  of  the  coulometer  is  particularly  reviewed  by  Guthe,  Bull.  No.  3, 
Bureau  of  Standards,  1905,  page  347.  and  by  Smith,  Mather  and  Lowrv.  Phil. 
Trans.,  207,  545.  Rayleigh  and  Sedgwick;  Phil.  Trans.,  175,  in  (1884).  ' 

2  Duschak  and   Hulett:   Trans.   Am.   Electrochem.    Soc.,   12,   257    (1907). 

3  Hering:   Trans.   Am.    Electrochem.    Soc.,    12,    293    (1907);    Bose:    Chem.   Ztg.,    26, 


67    (1902). 


18946C 


2  G.  D.  BUCKNER. 

information  on  this  question  could  be  obtained  by  using  as  a 
cathode  cup  some  metal  such  as  gold,  which  does  not  measurably 
absorb  oxygen. 

Considerable  work  has  been  done  on  the  silver  coulometer  in 
vacuo  and  in  solutions  saturated  with  nitrogen,  which  means 
the  exclusion  of  oxygen,  and,  while  the  evidence  is  not  con- 
clusive (most  experimenters  have  observed  a  heavier  deposit 
where  oxygen  has  been  excluded),  this  might  be  explained  by 
the  foregoing  assumption. 

The  type  of  coulometer  used  by  us  was  essentially  the  same 
as  that  used  by  Duschak  and  Hulett.  Two  of  the  cups  were 
made  of  gold  and  two  of  platinum,  each  containing  a  porous 
cup  to  retain  the  heavy  anode  liquid.  These  were  run  in  series, 
with  the  idea  of  comparing  the  silver  deposited  on  platinum  and 
on  gold,  in  order  to  determine  whether  gold  could  be  substituted 
for  platinum  as  a  cathode  and  thereby  overcome  the  influence 
which  might  be  caused  by  the  oxygen  in  the,  platinum. 

We  soon  found  that  the  purity  of  the  materials  used  in  making 
up  the  electrolyte  affected  the  deposits,  and  also  that  the  inclu- 
sions in  the  deposited  silver  had  to  be  taken  into  consideration, 
so  this  investigation  really  involved  several  important  questions. 
and  has  resulted  in  two  independent  researches.  Since  it  soon 
developed  that  the  slight  impurities  in  the  electrolyte  caused 
measurable  variations  in  the  weight  of  the  silver  deposited,  it 
seemed  probable  that  this  variation  in  the  weights  might  be  due 
to  the  variation  in  the  impurity  included  by  the  deposited  silver. 
A  very  careful  study  of  inclusions  in  electrolytic  silver  was  taken 
up  by  Mr.  J.  S.  Laird,  and  a  method  developed  for  the  accurate 
determination  of  these  inclusions,  as  described  in  a  separate  paper. 
Our  first  problem  was  to  get  a  standard  reproducible  silver  nitrate 
solution. 

Incidentally  we  obtained  some  information  from  the  suggestions 
of  Richards4  and  others  that  a  complex  ion  is  formed  at  the 
anode,  and  that  when  this  anolyte  comes  in  contact  with  the 
cathode  the  deposit  of  silver  is  too  heavy.  In  testing  this  point 
we  arranged  two  coulometers  in  series,  in  which  we  maintained 
by  the  use  of  syphons  the  catholyte  and  anolyte  at  such  levels 
that  in  each  cathode  cup  the  flow  of  the  liquid  would  be  towards 
the  opposite  pole. 

4  Richards.   Collins  and  Heimrod;   Proc.   Am.   Acad.,   35,    123    (1899). 


THE  SILVER  COULOMETER.  3 

We  also  attempted  to  substitute  "Alundum"  cups  made  of 
pure  A12O3  for  the  porous  porcelain  cups,  since  these  permitted 
an  easier  flow  of  liquid  and  lessened  the  resistance. 

In  series  with  most  of  these  experiments  we  ran  a  coulometer 
of  the  Rayleigh  type,  using  the  filter  paper  inclosed  anode.  We 
also  experimented  with  electrolytes  of  different  degrees  of  purity. 

Apparatus  Used. 

Each  cathode  cup  was  10  cm.  high,  5  cm.  in  diameter,  and 
weighed  72  grams.  The  cups  were  so  adjusted  as  to  weigh  within 
o.ooi  gram  of  each  other,  the  advantage  of  this  being  that  one 
tare  would  suffice  for  all  of  the  cups  and  the  exact  differences 
in  weight  could  be  determined  with  the  rider.  The  cathode  cups 
Nos.  i  and  2  were  of  platinum,  while  5  and  4  were  of  pure  gold. 
These  cathode  cups  were  handled  with  tongs  at  all  times.  They 
were  thoroughly  cleaned  with  sea  sand,  also  chemically,  and 
washed,  after  which  they  were  heated  to  redness  for  ten  minutes 
in  an  electric  furnace.  This  final  treatment  produced  a  surface 
on  the  gold  cups  to  which  the  silver  adhered  so  tenaciously  that 
it  could  not  be  readily  detached.  Since  the  deposits  were  to  be 
saved,  this  treatment  was  abandoned.  When,  however,  the 
cathode  cups  were  placed  in  wooden  molds,  and  the  internal 
surfaces  made  smooth  with  a  blood-stone  burnisher,  then  cleaned 
as  stated  above  and  heated  in  an  electric  furnace  at  160°  for 
thirty  minutes,  it  was  found  that  this:  treatment  gave  a  gold  sur- 
face from  which  the  silver  deposits  could  generally  be  removed 
with  a  platinum  spatula.  The  deposits  in  the  platinum  cups  could 
always  be  removed  with  ease. 

The  weighings  were  made  under  the  same  conditions  as  those 
described  by  Duschak  and  Hulett,  the  only  difference  being 
that  the  swing  divisions  were  read  through  a  small  telescope 
fixed  in  the  balance  case.  Duplicate  weighings  could  be  made 
to  0.02  mg.  with  certainty.  Care  was  always  taken  that  tem- 
perature equilibrium  had  been  established  in  the  balance  case, 
and  the  cups  were  always  placed  on  the  pans  with  long  tongs. 
The  weights  were  calibrated,  and  at  each  weighing  the  tempera- 
ture, barometer  and  hygrometer  were  noted.  The  anodes  were 
made  of  "atomic-weight"  silver,  the  purification  of  which  is 
described  elsewhere.  The  silver  was  melted  in  a  porcelain  cru- 


G.  D.  BUCKNER. 


cible  and  cast  into  cylindrical  sticks  6  cm.  by  i  cm.  in  a  mold 
made  of  pure  graphite  which  had  previously  been  heated  to 
redness  for  thirty  minutes.  These  anodes  were  suspended  by 
platinum  wires  which  served  as  electrical  connections.  The  four 
cathode  cups  were  cleaned,  adjusted  to  the  same  weight  by  re- 
moving a  little  from  the  top  of  the  heavier  ones,  cleaned  as 
described  above,  cooled  in  separate  vacuum  desiccators,  and 
finally  weighed. 


=:-_•=-•= 
___ 


FIG.  i. 


The  cathode  cups  were  now  set  up  ready  for  a  run,  on  circular 
pieces  of  plate  glass  the  bottoms  of  which  had  been  previously 
paraffined  and  on  the  top  of  which  had  been  placed  several 
pieces  of  filter  paper  to  insure  perfect  insulation.  The  cathode 
cups  (a)  as  shown  in  Fig.  i  were  surrounded  by  glass  jackets  (b ) 
which  extended  2.5  cm.  above  the  cups,  and  the  glass  cylinders 
were  covered  with  pieces  of  glass  provided  with  two  holes,  those 
in  the  center  being  just  large  enough  to  hold  the  porous  cups  (c), 


THE  SILVER  COULOMETER.  5 

while  those  at  the  side  and  through  which  the  catholyte  was  intro- 
duced were  smaller.  On  the  glass  cover  rested  a  hard  rubber 
bridge  (d)  from  which  the  anode  (e)  was  suspended  as  indicated 
in  the  figure.  In  series  with  these  four  coulometers  we  ran  one 
of  the  Rayleigh  type,  modeled  after  the  specifications  prepared 
by  the  National  Academy  of  France.  The  Rayleigh  bowl  was 
9  cm.  by  4  cm.  deep. 

The  porous  cups  were  exact  copies  in  size  and  texture  of  the 
Berlin  "Pukel"  filter  tubes  except  that  the  upper  3  cm.  were 
vitreous.  They  were  2.5  cm.  in  diameter  by  12  cm.  long,  and 
when  placed  in  position  extended  to  within  0.75  cm.  of  the  bottom 
of  the  cathode  cups.  These  porous  cups  were  treated  with  con- 
centrated nitric  acid,  and  then  water  was  allowed  to  flow  through 
them  continuously  until  the  last  trace  of  nitric  acid  had  been 
removed.  At  no  time  were  they  glowed  or  allowed  to  dry  out. 
During  a  run  the  anolyte  was  removed  at  intervals  with  a  constant 
level  pipette  and  the  catholyte  added,  so  that  the  electrolyte  was 
always  flowing  into  the  porous  cups  towards  the  anode.  After 
an  experiment  had  been  completed  the  silver  crystals  were 
loosened  with  a  platinum  spatula  and  sealed  in  test  tubes  which 
had  previously  been  glowed.  This  silver  was  examined  by  Mr. 
Laird,  as  described  in  another  communication. 

Materials  Used. 

In  previous  work  \ve  learned  that  filter  paper  was  to  be  avoided, 
and  in  the  present  investigation  we  soon  found  that  not  only  filter 
paper,  but  dust,  organic  matter  of  every  kind  and  even  dissolved 
glass  so  affected  a  silver  nitrate  solution  that  it  gave  abnormally 
heavy  deposits.  This  research  is  therefore  largely  devoted  to  the 
question  of  the  purity  of  the  materials  used  in  coulometer  work. 

In  order  to  have  a  reliable  and  reproducible  basis  of  com- 
parison it  was  found  necessary  to  work  with  a  silver  nitrate 
solution  made  in  a  definite  way,  which  was  essentially  the  same 
as  that  used  by  Richards  in  preparing  the  silver  nitrate  from 
which  he  determined  the  atomic  weight  of  silver.  This  must 
give  us  a  solution  in  which  the  equivalence  between  Ag  and  NO3 
is  exactly  equal,  provided  we  rigorously  exclude  organic  matter 
in  all  subsequent  operations.  We  therefore  paid  particular  atten- 
tion to  the  elimination  of  all  organic  matter  from  our  water,  and 


6  ('..   I).   HL'CKX  KK. 

used  only  quartz,  gold  and  platinum  vessels,  so  that  there  could 
be  no  possible  reduction  of  the  silver  nitrate  by  dissolved  glass 
or  by  any  other  reducing  agent.  This  is  our  "normal"  coulometer 
electrolyte. 

Preparation  of  Silver. 

The  first  step  in  purifying  silver  is  to  obtain  it  in  the  form  of 
silver  chloride.  Richards5  digested  silver  chloride  in  a  solution 
of  potassium  hydroxide  and  sugar,  in  order  to  obtain  metallic 
silver.  If,  however,  the  silver  chloride  is  dissolved  in  ammonium 
hydroxide  and  filtered,  certain  substances  are  eliminated,  and 
there  is  an  advantage  in  reducing  silver  directly  from  a  solution 
rather  than  from  the  solid.  We  encountered  a  difficulty  in  reduc- 
ing the  silver  from  an  ammoniacal  solution  with  hydroxide  and 
sugar  in  a  desirable  crystalline  form,  but  found  that  this  could 
be  done  as  follows :  Silver  was  obtained  from  the  U.  S.  Mint, 
dissolved  in  nitric  acid  and  precipitated  as  silver  chloride.  It 
was  then  digested  in  aqua  regia  for  several  hours  on  the  water 
bath,  filtered  and  washed.  The  silver  chloride  was  dissolved 
nearly  to  saturation  in  a  closed  container  in  ammonium  hydroxide 
of  specific  gravity  0.93.  Five  hundred  c.c.  of  this  silver- 
ammonium  chloride  solution  was  then  decanted  and  filtered  into 
a  liter  flask  which  contained  25  g.  of  cane  sugar  dissolved  in 
50  c.c.  of  water.  Silver  did  not  appear  at  first,  but  when  thes 
solution  was  heated  to  incipient  boiling  the  silver  began  to  separate 
in  a  very  finely  crystalline  form.  The  ammonia  which  distilled 
was  led  into  pure  water  for  future  use.  As  the  reduction  pro- 
ceeded a  solution  containing  100  g.  of  sugar  and  25  g.  of  potas- 
sium hydrate  in  150  c.c.  of  water  was  added  through  a  dropping 
funnel,  rapidly  at  first  and  more  slowly  as  the  evolution  of 
ammonia  becomes  less.  After  the  sugar  solution  had  been  added 
and  the  evolution  of  ammonia  had  ceased,  the  liquid  was  vigor- 
ously boiled  for  fifteen  minutes.  Under  these  conditions  we 
were  able  to  reduce  silver  from  an  ammoniacal  solution  in  a 
crystalline  form  which  could  be  readily  washed.  It  was  found 
to  be  finely  granular,  and  contained  only  exceptionally  the 
slightest 'traces  of  silver  .chloride,  which  were  entirely  removed 
by  shaking  thoroughly  with  ammonium  hydroxide.  The  small- 
grained  silver  possessed  the  virtue  of  lessening  the  possibility  of 

•"•Richards  and  Wells;  Jour.   Chem.   Soc.,   27,  475    (1905). 


THE  SILVER  COULOMETER.  7 

silver  chloride  and  other  impurities  being  occluded,  it  was  easily 
soluble  in  nitric  acid,  and  the  solution  gave  no  cloudiness  on 
dilution.  The  silver  was  now  placed  in  a  Jena  flask,  dissolved 
in  nitric  acid,  the  solution  evaporated,  and  the  residue  fused  in  an 
electric  furnace.  The  fused  silver  nitrate  was  dissolved  in  "con- 
ductivity" water,  and  the  metal  precipitated  with  ammonium 
formate  according  to  the  directions  of  Richards  and  Wells.  This 
silver  was  composed  of  large  crystals,  and  was  as  pure  as  "atomic- 
weight"  silver.  It  was  melted  and  cast  into  molds,  as  has  been 
already  described.  This  silver  was  used  in  the  preparation  of 
our  "normal"  electrolyte. 

Purification  of  Water. 

Since  dust,  organic  matter  or  dissolved  glass  in  the  water  would 
partially  reduce  our  purest  silver  nitrate,  particular  care  was 
exercised  in  purifying  the  water  employed  in  our  work.  The 
apparatus  consisted  of  a  1 5-liter  Jena  retort,  the  neck  of  which 
was  drawn  down  and  cut  off  so  as  to  just  fit  a  quartz  condenser 
tube  ground  into  and  extending  into  the  retort.  The  arrange- 
ment was  such  that  only  vapor  could  enter  the  condenser,  the 
water  separating  on  the  glass  simply  flowing  back  into  the  boiling 
liquid.  Air  filtered  through  absorbent  cotton  and  deprived  of  all 
organic  matter  by  passage  over  a  glowing  platinum  spiral,  wound 
evenly  on  a  porcelain  tube,  was  bubbled  slowly  through  the  gently 
boiling  water.  This  water  contained  a  small  amount  of  KMnO4 
and  Ba(OH)2  and  was  freshly  prepared  conductivity  water. 
The  whole  apparatus  was  drained  for  fifteen  minutes  before  any 
water  was  condensed,  only  the  second  third  being  caught  and 
retained  in  a  quartz  flask.  Since  this  water  had  been  in  contact 
with  quartz  only,  it  must  have  been  free  from  all  traces  of  dust 
and  organic  matter. 

Preparation  of  Silver  Nitrate  and  Nitric  Acid. 

The  nitric  acid  was  purified  according  to  a  method  identical 
in  the  main  to  the  one  employed  in  obtaining  pure  water.  No 
air,  however,  was  bubbled  through  the  acid  during  its  distillation. 
The  purest  silver  was  dissolved  in  this  nitric  acid  and  water 
prepared  by  the  foregoing  method.  The  quartz  flask  was  placed 
in  an  electric  furnace  where  the  temperature  was  accurately'  con- 


8  G.  D.  BUCKNER. 

trolled,  the  solution  was  evaporated,  and  the  residue  fused  ac- 
cording to  Richards'  directions  until  the  last  trace  of  nitric  acid 
had  been  expelled.  Extending  down  into  the  quartz  flask  which 
contained  the  silver  nitrate  was  a  porcelain  tube  through  which 
passed  air  purified  as  previously  described.  The  silver  nitrate 
was  kept  covered,  in  a  dark  place,  until  used  the  day  following, 
and  the  water  was  prepared  and  the  solution  made  just  before  the 
experiment  was  started.  A  15  percent  solution  was  used  in  all 
experiments.  We  regarded  this  as  the  purest  obtainable  silver 
nitrate  solution  on  a  reproducible  basis. 

Manipulation. 

After  obtaining  the  accurate  weights  of  the  cathode  cups  they 
were  placed  on  the  circular  glasses  and  filter  papers  with  a  strip 
of  platinum  foil  under  each  cup,  which  served  as  a  conductor 
for  the  electric  current.  The  glass  jackets  were  placed  over 
them,  and  the  silver  nitrate  solution,  prepared  as  above,  was 
added.  The  porous  cups  were  now  rinsed  with  silver  nitrate 
solution  and  placed  in  position;  thus  the  catholyte  began  im- 
mediately to  diffuse  into  them.  The  anodes  were  fixed  in  place, 
and  then  the  anolyte  was  added,  but  only  to  such  a  level  that 
the  catholyte  was  always  passing  into  the  anode  chamber.  After 
a  current  of  about  0.3  ampere  had  passed  through  the  coulometers 
for  about  four  hours,  depositing  approximately  5  grams  of  silver 
in  each  cup,  the  current  was  broken  and  the  anodes  were  removed. 
The  anolyte  was  drawn  off  with  a  pipette,  and  following  this 
the  catholyte  was  removed  by  the  same  method.  Finally  the 
porous  cups  were  removed  and  examined  for  small  particles 
of  loose  silver  which  might  have  adhered  to  them.  The  anolyte 
and  catholyte  were  reserved  in  separate  flasks,  and,  although 
all  solutions  were  tested  for  acidity,  they  were  always  found  to 
be  neutral. 

The  cathode  cups  were  allowed  to  drain  into  small  crystallizing 
dishes,  each  inclined  against  a  glass  support.  The  cups  were 
next  filled  with  distilled  water,  which  was  removed  with  a  freshly- 
blown  bulb  pipette,  run  into  a  marked  flask,  and  the  cathode 
cups  were  drained  again  into  the  same  crystallizing  dishes.  This 
was  repeated  three  times,  until  the  water  which  had  stood  over 
night  in  the  cups  gave  no  test  for  silver  with  KI.  It  was  quite 


THE  SILVER  COULOMETER. 


impossible  for  any  loose  silver  to  escape  us  in  this  way,  for 
previously  it  was  shown  that  as  little  as  0.002  mg.2  of  silver 
could  be  detected  in  a  flask  and  recovered.  The  cathode  cups 
were  always  handled  with  tongs,  wound  with  linen  thread,  so 
that  no  dust  or  liquids  came  in  contact  with  the  outside  of  the 
cups,  which  were  always  bright  and  polished.  The  loose  silver 
was  placed  in  the  proper  cathode  cups,  and  these  were  dried  in 
vacuum  desiccators  and  weighed.  We  also  satisfied  ourselves  that 
there  was  no  appreciable  change  in  weight  when  the  cathode  cups 
with  the  deposits  were  subsequently  heated  to  160°  for  thirty 
minutes.  In  the  experiments  made  using  the  two  platinum  and 
two  gold  cathode  cups  a  "Rayleigh"  form  was  also  run  in  the 
series.  The  results  are  as  follows  (Table  I)  : 

TABLE  I. 


Run 

On 

Pt. 

Differ- 

On 

AU 

Differ- 

No. 

i 

2 

ing. 

3 

4 

mg. 

7 
8 
9 
10 
ii 

5.83369 
547605 
5-59653 
4.98000 
6.26536 

5.83383 
547586 
5-59663 

4-97953 
6.26540 

0.14 
O.I9 
0.10 

0-47 
0.04 

5-83449 

lost 

5-59756 
4.98036 
6.26589 

5-83444 
5-47637 
5-59746 
4.98038 
6.26603 

•05 

.10 
.02 
.14 

Deposit  on 

Rayleigh 

Run          Average 

Average 

'Rayleigh'        Gold  heavier  |         deposit 

No.         Pt.  deposit 

Au  deposit 

deposit                than  on 

heavier  than 

Pt.  mg. 

on  Pt.  mg. 

7 

5.83376 

5-83446 

5.83525 

+0.70 

+1-49 

8 

5-47595 

547637 

5-47749 

+0.42 

-f-  1  -54 

9 

5.59658 

5-59751 

5.59848 

+0.93 

4-1.90 

IO 

4.97976 

4.98037 

4.98184 

-fo.6i 

+2.08 

II 

6.26538 

6.26596 

6.26661 

+0.58 

+1.23 

If  we  calculate  the  percentage  difference  in  the  weight  of 
silver  deposited  on  platinum  and  on  gold  by  the  same  current, 
we  find  that  the  deposits  on  the  gold  are  0.012  ±  0.0012  percent 
heavier  than  the  deposits  on  platinum,  while  in  the  Rayleigh 
coulometer,  where  Kahlbaum's  C.  P.  silver  nitrate  and  the  filter 
paper  enclosed  anode  were  used,  we  found  the  deposits  to  be  0.030 
±.  0.0033  percent  heavier  than  the  deposits  made  on  the  normal 
platinum  cathode  cup.  In  every  case  it  was  found  that  the  silver 


10  G.  D.  BUCKNER. 

deposited  on  gold  was  heavier  than  that  deposited  on  platinum 
by  the  same  current,  the  average  excess  being  12  parts  in  100,000. 
This  could  be  explained  by  assuming  that  part  of  the  current  in 
the  platinum  coulometers  passed  from  the  catholyte  to  the  plat- 
inum cathode  by  removing  oxygen  from  the  platinum. 

The  silver  samples  reserved  from  these  runs  were  investigated 
by  Mr.  Laird,  but  before  the  method  of  determining  the  conclu- 
sions was  in  its  final  state,  the  deposits  from  the  gold  cups 
showed  in  all  cases  slightly  more  inclusions  and  those  from  the 
Rayleigh  form  very  much  more  than  did  the  deposits  from  the 
platinum  cups.  The  difference  in  the  amounts  of  inclusions  in 
the  silver  from  the  platinum  and  gold  may  be  due  to  the  fact 
that  the  silver  crystals  deposited  on  the  gold  were  close  together 
and  nearly  covered  with  gold,  whereas  those  on  the  platinum 
were  isolated. 

These  deposits  on  the  platinum  also  contained  impurities,  so 
in  our  judgment  it  matters  little  which  metal  we  use  as  cathode 
when  the  exact  electrochemical  equivalent  of  silver  is  to  be  deter- 
mined, for  the  impurities  in  the  silver  deposit  must  be  determined 
in  any  case  if  an  accuracy  greater  than  i  in  5,000  is  desired. 

In  examining  the  deposits  we  found  that  the  silver  crystals 
deposited  on  the  platinum  cathode  were  rather  large  and  well 
defined,  being  placed  irregularly  on  the  sides  of  the  cathode  cups, 
with  only  a  few  on  the  bottom.  Only  in  very  few  cases  was 
there  a  tendency  towards  a  striated  arrangement  of  the  crystals 
on  the  platinum,  and  in  all  cases  the  silver  was  easily  detached 
by  a  platinum  spatula  without  appreciably  changing  the  weight 
of  the  cups.  One  of  the  difficulties  encountered  in  using  a 
platinum  cathode  was  the  fact  that  there  was  always  a  small 
amount  of  loose  silver  present  which  had  to  be  recovered. 

It  seemed  to  us  that  gold  might  be  a  more  desirable  metal 
for  the  cathode  cups  than  platinum,  and  when  the  surface  of  the 
gold  cups  was  slightly  rough  there  was  generally  no  loose  silver, 
but  sometimes  there  was,  for  the  adhesion  of  the  silver  crystals 
to  the  gold  cups  was  not  sufficient  invariably  to  retain  all  of 
the  silver  during  the  manipulations.  However,  the  adhesion  was 
sufficient  to  make  it  a  very  difficult  matter  to  remove  the  silver 
for  examination  as  to  inclusions.  Gold  is  a  soft  metal,  and  the 
thin-walled  cups  were  difficult  to  handle.  Since  the  silver  crystals 


THE  SILVER  COULOMETER.  1 1 

completely  covered  certain  parts  of  the  gold  cathodes,  and  there 
may  have  been  more  chance  for  inclusions  between  the  silver 
and  gold  than  between  the  silver  and  platinum  in  the  platinum 
cathodes,  the  platinum  cups  appear  on  the  whole  to  be  superior 
to  those  of  gold,  and  if  the  dissolved  oxygen  plays  any  role 
it  is  only  a  few  parts  in  100,000. 

The  deposits  in  the  "Rayleigh"  form  were  feathery  and 
striated  at  all  times.  The  comparative  character  of  the  deposits 
in  the  platinum  and  gold  cups  can  be  seen  in  Fig.  2. 


FIG.   2.     The  gold  cup  on  the  right  shows  the  small  Ag  crystals  as  compared  with 
those  deposited  on  a  platinum  cup  as  shown  on  the  left. 


The  Effect  of  Solutions  of  Knoivn  Purity. 

Since  the  impurities  in  the  deposits  caused  the  abnormal 
weights,  we  decided  to  try  some  experiments  in  which  the  silver 
nitrate  solutions  in  the  cathode  cups  were  of  different  but  of 
known  sources.  In  platinum  cup  No.  I  we  used  our  "normal" 
solutions,  in  No.  2  a  solution  of  Kahlbaum's  C.  P.  silver  nitrate 
which  had  been  recrystallized  after  adding  I  c.c.  of  our  purest 
nitric  acid  to  the  solution,  decanting,  evaporating  and  fusing  the 
salt  as  directed  above.  The  manipulation  was  carried  out  in 
detail  as  in  the  previous  experiment.  In  the  last  four  experiments 
we  also  ran  a  "Rayleigh"  type  which  contained  the  same  re- 
crystallized  silver  nitrate  solution  contained  in  cup  No.  2.  The 
results  were  as  follows : 


12 


G.  D.  BUCKNER. 

TABUS  II. 


Run 
No. 

"Normal" 

A 
ing. 

Kahlbaum's 

mg.   Heavier 
than  Normal 

Rayleigh 
Kahlbaum's 

12 
13 
14 
15 

16 

17 

18 
19 

20 

7-I2I75 
4-96636 
5.97746 

6.66X93 
572269 
6.04IOI 
5-040/3 
5.77169 

0.58 
0.76 
0.82 

0.29 

1.61 
0.41 
0.30 
—1.09 

6^66222 
5-72430 
6.04142 
5.04103 
5-77060 

2.99 
3-19 
1-53 
3-18 
2.09 

6.66492 

572588 
6.04254 
5.0439I 
5-77378 

In  experiment  20  the  silver  deposit  in  coulometer  No.  2  was 
partially  discolored,  and,  while  the  cause  is  unknown,  it  was 
obviously  to  be  rejected,  although  we  proceeded  with  the  weigh- 
ing as  a  matter  of  form.  Referring  to  table  No.  2,  coulometer 
No.  i  always  contained  our  "normal"  silver  nitrate  solution,  while 
in  No.  2,  which  was  like  No.  I  in  all  other  respects,  was  placed 
the  solution  made  from  our  best  water  and  Kahlbaum's  silver 
nitrate,  which  had  been  treated  with  a  little  nitric  acid,  re- 
crystallized  and  fused  as  previously  described.  This  same  solu- 
tion was  also  used  in  the  Rayleigh  form.  We  expected  to  find 
that  the  Kahlbaum's  C.  P.  silver  nitrate  when  treated  in  this  way 
would  give  the  same  results  as  our  "normal"  solution,  but  we 
found  that  coulometer  No.  2  in  every  case  gave  a  heavier  deposit, 
the  average  excess  being  0.009  percent.  We  noticed  also  that 
the  Rayleigh  form  with  the  same  solution  as  that  in  coulometer 
No.  2  gave  a  deposit  which  was  always  much  heavier  than  that 
obtained  from  our  normal  electrolyte,  the  average  excess  being 
0.036  percent.  Furthermore,  the  deposits  from  coulometer  No.  2 
show  distinctly  more  inclusions  than  those  from  coulometer  No.  I, 
while  the  deposits  from  the  Rayleigh  show  still  greater  amounts 
of  inclusions  than  from  No.  2. 

It  would  follow,  therefore,  that  the  source  and  previous  history 
of  the  silver  nitrate  solution  always  plays  a  measurable  role 
in  the  weight  of  silver  deposited  by  a  given  number  of  coulombs. 
We  are  therefore  forced  to  the  conclusion  that  it  is  necessary  to 
determine  and  allow  for  the  inclusions  in  the  deposited  silver 
when  any  attempt  at  great  accuracy  is  made,  and,  although 


THE  SILVER  COULOMETER.  13 

reasonably  pure  materials  for  the  electrolytes  will  suffice,  the 
purer  the  solutions  used  the  less  the  error  due  to  inclusions, 
which  is  as  small  as  one  part  in  10,000  for  our  "normal"  electro- 
lyte in  a  porous  cup  platinum  coulometer,  where  the  anolyte 
has  no  access  to  'the  catholyte  during  the  electrolysis. 

Effect  Due  to  the  Anolyte. 

It  has  been  maintained  by  Richards  and  others  that  the  solu- 
tion formed  at  the  silver  anode  in  a  silver  nitrate  solution  is 
abnormal  in,  that  it  will  yield  an  abnormally  heavy  deposit  when 
used  as  catholyte.  Table  II  clearly  shows  that  the  Rayleigh 
form  gives  a  markedly  heavier  deposit  than  does  the  porous  cup 
coulometer  with  the  same  solution.  Filter  paper  has  the  property 
of  so  changing  a  silver  nitrate  solution  that  it  gives  a  heavier 
deposit,  a  fact  which  in  part  or  wholly  would  account  for  the 
difference  between  the  results  obtained  with  coulometer  No.  2 
and  the  Rayleigh  form,  as  shown  in  Table  II.  There  is  another 
possible  factor.  In  the  Rayleigh  coulometer  the  silver  nitrate 
formed  at  the  anode  increases  the  density  of  the  solution  at  the 
bowl,  and  it  flows  down  through  the  filter  paper  and  comes  in 
contact  with  the  cathode.  In  order  to  get  some  evidence  on  the 
possible  effect  of  the  anolyte  we  carried  out  the  following 
experiments : 

In  our  porous  cup  coulometers  /,  2,  5  and  4,  filter  paper  and 
all  organic  matter  were  excluded;  furthermore,  the  anolytes  in 
the  porous  cups  were  always  maintained  at  a  lower  level  than 
the  catholytes,  so  that  the  flow  of  liquid  was  from  the  catholyte 
to  the  anolyte.  Now,  by  keeping  the  electrolyte  at  a  higher 
level  in  the  anode  than  in  the  cathode  cups,  we  can  cause  the 
anolyte  to  flow  through  the  porous  cups  into  the  catholyte. 

We  ran  two  coulometers  in  series.  From  No.  I  we  removed 
the  anolyte  by  a  constant  level  syphon  which  extended  to  the 
bottom  of  the  porcelain  cup,  thereby  removing  the  heaviest  liquid 
as  it  collected  at  the  bottom.  In  No.  2  the  catholyte  was  removed 
by  a  quartz  syphon  which  only  extended  below  the  top  of  the 
catholyte,  thereby  removing  only  the  lighter  and  allowing  the 
heavy  liquid  to  flow  through  the  porous  cup  and  collect  at  the 
bottom  of  the  cathode  cup.  The  electrolytes  were  our  "normal" 
silver  nitrate  solutions,  and  all  details  of  the  two  coulometers 


G.  D.  BUCKNER. 


were  identical  except  the  direction  of  the  flow  of  the  electrolyte. 
The  results  were  as  follows : 

TABLE  III. 


Run 

Catholyte 

Anolyte 

Difference 

No. 

removed 

removed 

mg. 

32 

4.66l8l 

4.66079 

+1.02 

33 

4-I3056 

4.12998 

+0.58 

34 

3-75443 

3-75361 

+0.82 

35 

5-12350 

5.12286 

+0.64 

36 

4.64026 

4.63926 

+  1.00 

These  results  show  that  the  heavy  anolyte  does  play  some  role 
in  the  formation  of  the  heavier  deposits  obtained.  These  five 
results  indicate  an  increase  in  weight  of  the  silver  deposited  of 
0.019  percent,  due  to  the  presence  of  the  anolyte  during  elec- 
trolysis. This  is  approximately  half  of  the  increase  in  weight 
(0.036  percent)  observed  in  the  Rayleigh  form,  where  the  anolyte 
not  only  flows  into  the  catholyte,  but  filter  paper  also  is  present. 

Porous  Cups  of  Different  Material. 

The  porous  cups  with  vitreous  upper  portion,  which  we  used, 
were  entirely  satisfactory.  They  offered,  however,  more  resist- 
ance to  the  current  than  was  desired,  and  it  took  a  considerable 
length  of  time  to  wash  them  absolutely  free  of  acids.  For  other 
reasons  it  seemed  desirable  also  to  test  porous  cups  made  of  some 
other  material  than  porcelain,  and  we  selected  "alundum,"  which 
can  be  made  into  excellent  filtering  tubes.  It  did  not  seem  con- 
ceivable that  crystalline  alumina  could  affect  a  silver  nitrate  solu- 
tion. These  tubes  were  of  the  same  dimensions  as  the  porous 
cups,  only  thin  walled  (1.5  mm.),  allowed  the  liquid  to  filter 
through  much  more  readily,  and  could  be  washed  thoroughly  in  a 
short  time.  Four  alundum  cups  having  the  above  dimensions 
were  obtained  and  thoroughly  cleaned  with  nitric  acid  and  water. 
Experiments  21,  22,  23  and  24  of  Table  IV  were  made  with  these 
substitutes  for  the  porous  porcelain  cups,  the  normal  electrolyte 
being  used.  The  catholyte  and  anolyte  were  maintained  at  the 
same  level.  Here  the  agreement  of  the  results  obtained  with  the 
platinum  cups  No.  /  and  2  is  particularly  good,  and  a  comparison 


THE  SILVER  COULOMETER. 


of  the  results  obtained  with  the  gold  cups  3  and  4  is  also  good; 
but  here  again  we  find  the  heavier  deposits  in  the  gold  cups.  In 
run  23  there  was  a  very  fine  scum  floating  on  the  catholyte  in 
the  platinum  cups  which  could  not  be  accounted  for.  The  run, 
however,  was  completed  as  in  all  cases.  These  alundum  cups 
retained  the  anode  slime  perfectly,  but  are  quite  porous,  and  no 
doubt  some  of  the  anolyte  diffused  through  them  to  the  catholyte. 
We  therefore  made  six  experiments  with  the  two  platinum  cups. 
Coulometer  No.  /  was  provided  with  an  alundum  porous  cup 
in  which  the  anolyte  was  at  the  same  level  as  that  in  the  cathode 
chamber,  while  No.  2  contained  the  original  porcelain  cup  where 
the  anolyte  was  at  a  lower  level  and  there  was  no  diffusion  in  to 
the  catholyte.  In  every  case  except  in  27  the  coulometer  with 
the  alundum  cup  showed  a  heavier  deposit.  The  average  is  0.026 
percent.  The  results  are  as  follows  : 

TABU;  IV. 


Run 
No. 

i 
Alundum 

Differ- 
ence 
mg. 

2 

Alundum 

Gold  heav- 
ier than 
Plat.  mg. 

Alundum 

Differ- 
ence 
mg. 

4 
Alundum 

21 
22 
23 
24 

2<5 

5.88568 
5-33804 
5-7II43 
4.42802 

4.71515 

0.31 
•05 
•05 
-03 

-j-o  03 

5-88537 
5-33809 
57II38 
4-42799 
Porcel'n 
4  71518 

+2.06 

+0.59 

—0.49 
+0.26 

5.88766 

5  33874 
5.71104 
4.42820 

0.16 
0.16 

0.22 
O.06 

5-88750 
5.33858 
571082 
4.42826 

ofi 

4  92720 

-\-o  19 

4  92706 

27 

4.36494 

—  o  14 

4  36508 

"8 

4  52397 

-J-o  28 

29 

5.11922 

-j-o  31 

5  i  1891 

30 

3.83131 

+O.II 

3  83120 

The  effect  here  is,  no  doubt,  due  to  the  anolyte  diffusing 
through  the  porous  alundum  cup,  and,  while  we  could  arrange 
syphons  to  keep  it  slightly  lower  and  cause  the  flow  to  be  into 
the  anolyte,  the<  use  of  syphons  makes  the  apparatus  complicated 
and  causes  the  failure  of  a  larger  percent  of  the  experiments 
than  occurs  when  the  denser,  less  porous  porcelain  cups  are 
employed,  where  it  sufficed  to  remove  a  little  of  the  anolyte  with 
a  small  bulb  pipette  from  time  to  time,  which  is  a  simple  opera- 
tion, and,  besides,  there  was  no  diffusion  through  the  cup. 
Alundum  cups  could  be  made  of  the  same  texture  as  the  porcelain 


16  G.  D.  BUCKNEJR. 

cups,  but  then  they  would  lose  the  advantage  of  being  easily 
washed.  Our  porcelain  cups  with  vitreous  upper  portions  were 
distinctly  superior  to  the  "alundum"  cups,  and  the  coulometers 
as  used  were  made  for  simplicity  and  exactness  in  manipulation, 
and  were  quite  free  from  sources  of  accidental  error. 

General  Summary. 

1.  It  was  found  that  the  slightest  traces  of  impurities,  such  as 
dust  and  organic  matter,  so  affected  a  silver  nitrate  solution  that 
it  gave  abnormally  heavy  deposits.    Particular  attention  was  paid 
to  preparing  a  reproducible  "normal  electrolyte."  It  was  necessary 
to  take  unusual  precaution  in  preparing  all  materials,  and  we  used 
only  quartz,  gold  and  platinum  utensils. 

2.  In  preparing  "atomic-weight"  silver  the  usual  method  was 
modified.     We  found  the  conditions  under  which  silver  could 
be  precipitated  from  an  ammoniacal  solution  of  silver  chloride  in 
a  crystalline  condition,  which  is  a  distinct  advantage  over  reducing 
solid  silver  chloride  with  sugar  and  caustic  potash. 

3.  In  preparing  pure  water,  particular  attention  was  paid  to 
the  exclusion  of  dust  and  organic  matter  of  all  kinds,  even  to  the 
burning  of  volatile  organic  matter  which  might  be  present  in 
the  air  that  came  in  contact  with  the  solutions  used.     Since  the 
solubility  of  glass  suffices  to  affect  measurably  a  silver  nitrate 
solution,  the  water  employed  in  our  work  was  only  permitted  to 
come  in  contact  with  quartz. 

4.  Silver  nitrate  was  obtained   from  "atomic-weight"  silver 
and  pure  nitric  acid,  the  excess  of  acid  being  driven  off  at  a 
definite  temperature  in  an  electric  furnace,  and  with  the  exclusion 
of  all  organic  matter.     With  this  silver  nitrate  a  solution  was 
prepared  which  contained   exact  equivalents   of  Ag  and  NO3. 
Kahlbaum's  C.  P.  silver  nitrate,  recrystallized  and  fused,  gave  a 
deposit  0.009  percent  heavier  than  that  obtained  with  our  normal 
electrolyte,  and  other  solutions  yielded  distinctly  heavier  deposits. 

5.  Four  porous-cup   coulometers   were   run   in   series.     The 
cathode  cups  in  I  and  2  were  of  platinum,  while  3  and  4  were 
gold.     They  contained  the  same  solutions  and  were  treated  in 
exactly  the  same  manner.     In  every  case  we  found  the  deposits 
on  gold  to  be  heavier  than  those  on  platinum,  the  average  excess 
being  12  parts  in  100,000. 


THE  SILVER  COULOMETER.  17 

6.  In  experiments  with  a  "Rayleigh"   form  which  contained 
Kahlbaum's  C.  P.  silver  nitrate  solution,  when  run  in  series  with 
our  normal  coulometer,  the  Rayleigh  gave  a  deposit  0.036  percent 
heavy.    The  anode  of  the  coulometer  was  wrapped  in  filter  paper, 
and  the  anolyte  diffused  through  to  the  cathode  during  electrolysis. 

7.  In   our   porous-cup   coulometers   the  anolyte   was   always 
lower  than  the  catholyte,  and  the  former  did  not  flow1  through 
except  in  special  experiments,  which  showed  that  when  the  anolyte 
is  allowed  to  pass  through  the  porous  cup  and  come  into  contact 
with  the  cathode  a  deposit  heavier  by  0.019  percent  was  obtained. 
This  is  nearly  one-half  of  the  abnormal  effect  caused  by  the  heavy 
anode  liquid  and  filter  paper  as  shown  in  the  Rayleigh  form. 

8.  Experiments  were  made  with  thin-walled  porous  alumina 
cups   substituted    for   the   porcelain   cups.     The   silver   deposits 
made  using  the  alundum  cups  show  a  heavier  deposit  in  every 
case,  due  to  the  anolyte  passing  through  to  the  catholyte  during 
electrolysis. 

I  wish  to  express  my  great  obligation  to  Professor  G.  A.  Hulett 
for  having  suggested  this  research  and  for  his  kind  advice  and 
counsel  in  carrying  it  out. 

Laboratory  of  Physical  Chemistry, 
Princeton  University. 


189466 


This  book  is  DUE  on  the  last  date  stamped  below 

Dec  26  '58 


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